what i would like to do is to make the white chalk sketch (which is a layer in photoshop imported in AE) reveal starting from the first frame photo on the right, until the frame photo on the left. Exactly like if someone is writing or sketching something on a blackboard.

I really can't figure it out. With the Stroke Effect i m only able to overwrite the sketch but with a different brush, and i m not able to import my precious photoshop brushes in AE :( I was hoping there was a sort of mask that you can animate to reveal the object, instead of resketching it..

maybe you can help me ? someone told me to use the Stroke Effect with the option "Reveal original image" on, but i m not really sure i understood how to use that function..the sketch disappear and i don't know what to do.

This is one of those projects that can be accomplished in multiple ways, but here's one method you could use.

What you need is a graphics with multiple layers, chalk layer(top layer) and chalkboard/pictures for the background(bottom layer). So first set up your work in Photoshop and save as layered .psd file.

Import your graphic into AE as a composition(that option is in the bottom left corner of the import dialogue) and open up your composition.

On the top layer(chalk line) add the effect Generate -> Stroke. Now use the pen tool("G") to create a path for the stroke effect to follow. Start from where you want it to start revealing to the end. In the effect window, for paths choose "Mask 1"(assuming you haven't created any other masks in this composition. For Paint Style choose "Reveal Original Image".

Move the Current Time Indicator to where you want your animation to finish and press the Stroke "End" stop watch to set a key frame. Now move to where you want the animation to begin and change the percentage to 0%. This will set another key frame and if you scrub the time line you should see your chalk line animate on.

At this point you may need to thicken the brush size. If the stroke line is a little too smooth you could also add a "Stylize -> Roughen Edges" to it, that might make it look a little less smooth, but if the animation is fast enough, nobody is going to notice that.

Johnny Cuevas, Editor
Thinkck.com

"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.

thanks, your tricks are useful; i hoped to find a way to reveal the original sketch and not to resketching it, but maybe i ll be able to do it if i try to create again in AE nearly the same brush of the chalk. Thanks!